We are not to tell nature what she’s gotta be... She’s always got better imagination than we have.” -Richard Feynman

-Kaviranjana Antony

We live in a world that never fails to astound us. Today, we have one of the best tools at our disposal that help us understand the fundamental nature of matter and the 4 forces that govern them : The Standard Model. It encompasses 17 particles, matter particles which combine to form subatomic particles along with force particles, which mediate force interactions between the matter particles, thus forming the way we understand our universe at the most elementary level.

The Standard Model is undoubtedly the most accurate model we have, yet it still does not account for many unresolved mysteries. The main issue that plagues this chasm of ignorance is the irreconcilable nature of QFT and general relativity.

There are two fundamental theories we use to describe the universe. Quantum Field Theory explains electromagnetism, weak and strong nuclear forces along with the behaviour of sub-atomic particles by combining classical field theory, special relativity and quantum mechanics. On the other hand, we have General Relativity which describes gravity as a curvature of space-time and how objects interact with each other as a result. But we don’t know how to piece both these theories together.



QFT is unable to describe gravity using the particle physics framework while General Relativity says nothing about the quantum world. Well, why even bother unifying the both? Some of the most significant phenomena like the Big Bang and black holes are waiting to be understood fully through the fusing of these two theories.

This may even be the first step in the search for a grand unified theory i.e The Theory of Everything. The search for quantum gravity is severely limited by our current limits of technology, as the scale of particle accelerators and detectors would have to be of astronomical dimensions. Quantisation predicts the graviton, a fundamental particle that is responsible for gravitational interactions, however, our gravitational wave detectors simply cannot detect this particle, which is predicted to be smaller than Planck length.

Quantum physics describes matter at the scale of subatomic particles, where the effects of gravity are simply too small to show up. The electric repulsion between 2 electrons and their mutual gravitational attraction differ almost by the magnitude of 1042. This means that to measure quantum gravity, you would need incredibly high amounts of mass, compressed into the scale of subatomic particles. This results in exceedingly high energy situations which simply cannot be detected by our current gravitational wave detectors, LIGO and VIRGO.

This also serves as a useful explanation as to why postulated gravitions are so small-they essentially possess massive amounts of energy (E = h c / λ). String theory is a promising candidate. According to string theory, we have vibrating “strings” instead of particles and different vibrational modes give rise to different elementary particles. One of these vibrational modes is hypothesised to be the graviton. As always, there is a catch: there is no experimental evidence to back this prediction, there are multiple competing versions of the theory and it predicts the existence of many more hidden dimensions. This isn’t the only puzzle that modern physics fails to explain. There are many other conundrums that leave physicists stumped, for example the matter-antimatter asymmetry. Antimatter is essentially matter composed of anti-particles, which possess the same mass of the particle, but have opposite electric and magnetic properties. When a particle collides with its corresponding antiparticle, annihilation occurs, resulting in the formation of high-energy photons or in some cases, other fundamental particles.

When the Big Bang occurred, matter and antimatter were present in equal quantities. The matter and antimatter should have annihilated, resulting in a structure-less sea of photons in a matter- less void. There seems to have been an amount of matter which was preserved. But why? More importantly, where did all the antimatter go? Why does the universe today consist predominantly of matter rather than antimatter? We may never know the reason for this cosmic imbalance.

Almost 90% of the observable universe is made up of dark matter, which we know through its effect on the gravitational interactions between planets and galaxies. In the 20th century, physicists noted that the observations of galactic behaviour and experimental predictions were inconsistent with each other. They seemed to possess much more mass (about 400 times) than they actually did and were rotating at much higher speeds.

The speed depends directly on the amount of interior mass to the orbit; since luminous matter is concentrated at the centre of the galaxy, the speed decreases with the distance. Experimentally, it turned out that the speed remained almost constant even with increasing distance from the centre!

This would mean that there is some amount of matter distributed uniformly throughout the galaxy. This matter would not behave like luminous matter, rather it behaved like “invisible matter”, as it did not interact with light. This was later termed as dark matter and it does not interact with the electromagnetic field like normal matter.

This leads to an even more intriguing question: What actually is dark matter and what is it made up of? Weakly Interacting Massive Particles (WIMPs) are a promising candidate. WIMPs arise as a consequence of supersymmetry extensions in the Standard Model; supersymmetry postulates that every particle in the Standard Model has its corresponding “superpartner”. In that case, the lightest of these may be a WIMP. Another contender is the “axion”, which were supposedly created in the Big Bang and have very low masses, making them near impossible to produce or study.

It is a known fact that the universe is expanding, in fact, the rate of its expansion is increasing as well. The reason for this acceleration is attributed to dark energy, a form of energy that makes up almost 70% of the universe. Unsurprisingly, it was Einstein who proposed a similar concept i.e a “cosmological constant”, Λ, which describes the energy density of empty space. This theory gained relevance later on when we discovered that the expansion of the universe is accelerating. Dark energy and the cosmological constant seem to be consistent with each other when describing the zero-point energy of spacetime. However, there is a massive discrepancy (120 orders of magnitude) between the expected value of the cosmological constant and the one that is established by measurement, which goes to show we still have a long way to go when it comes to what dark energy actually is.

An alternative lies in modified Newtonian dynamics (MOND, for short) which suggests that gravity may have an inverse linear relation with distance, rather than the current inverse square law for accelerations lesser than a0, a new constant introduced by MOND. Although MOND explains galactice curve data excellently, it is unable to completely replace dark matter in many observations.

The world we live in is riddled with paradoxes and complexity to a scale we cannot even begin to imagine is a difficult thing to digest. Perhaps this is the beauty of science; sometimes we don’t get all the answers, we just learn how to ask better questions. Physicists are not quitters, so one day, we might just unravel all the mysteries of the universe in all its glory.

References:

Cover image credits: Lucas Taylor / CERN - http://cdsweb.cern.ch/record/628469

Image reference: Image courtesy of Symmetry magazine, a joint Fermilab/SLAC publication. Artwork by Sandbox Studio, Chicago.

Image reference Title: Galaxy rotation under the influence of dark matter.ogv

Author: Ingo Berg